2 Answers

That's a very subjective question because different moves are more powerful for different pokemon due to Same-Type-Attack-Bonus (STAB). A grass pokemon using Leaf Storm will have more power than them using Explosion.

As ohioscyther says Explosion has the highest base power at 250. Focus Punch, Giga Impact and Hyper Beam are next along with 150, but also have a side-effect (FP doesn't work if you were already hit, and the others make you immobile on the next turn).

There are also variable moves like Gyro Ball that can inflict up to 150 power. And Stealth Rock can take out 25% or even 50% of an opponent's HP when they switch in, without you doing anything!